In various facilities such as factories, leaked liquid way cause disasters or damage the facilities themselves, whereby it is necessary to detect the occurrence of leaked liquid at its early stage.
To this aim, there have conventionally been proposed various kinds of optical means which can detect whether leaked liquid have occurred or not. For example, a leak sensor using a filter made of thin paper or the like has been proposed.
This leak sensor emits light to the filter and detects the change in quantity of transmitted light or reflected light in the case where the filter is impregnated with a liquid, thereby determining whether a leaked liquid has occurred or not.
When a highly viscous liquid is subjected t o detection, however, the liquid is hard to infiltrate into the filter, thus making it difficult for the leak sensor using the filter to accurately grasp the change in light quantity, whereby leaked liquid is very hard to detect.
Also, since the filter has to be replaced upon every resetting operation, operators may suffer unexpected disasters upon replacing the filter when a highly dangerous liquid is subjected to detection, unless much attention is paid. Due to inconveniences as such, this kind of leak sensor has not been suitable for practical use.
Therefore, leak sensors using a light-transmitting part constituted by a transparent or semitransparent material in place of the filter made of thin paper or the like have been proposed.
In these leak sensors, for example, a casing is constructed by a transparent or semitransparent glass material, synthetic resin, or the like, while the bottom part thereof or the like is provided with the above-mentioned light-transmitting part.
For example, such a leak sensor utilizes the fact that, when probe light is incident from a medium having a refractive index n1 onto a medium having a refractive index n2 (n1&gt;n2 ), the reflectivity of probe light remarkably changes near its angle of reflection.
Namely, when no leaked liquid occurs, the difference in refractive index between the casing and the light-transmitting part is so large that the probe light having propagated through the casing is mostly reflected by the light-transmitting part and then propagates as reflected light.
If a leaked liquid adheres to the light-transmitting part, on the other hand, then the difference in refractive index between the casing and the light-transmitting part becomes smaller, so that the reflectivity at the boundary between the casing and the light-transmitting part lowers, whereby part of the probe light spills into the leaked liquid, by which the quantity of propagating reflected light decreases.
When a leak sensor employing the light-transmitting part as such is used, whether a leaked liquid has occurred or not can be detected while avoiding the shortcomings of the leak sensor using the filter.
Meanwhile, there are cases where the occurrence of a leaked liquid cannot be detected favorably if a casing is installed as it is in a subject of leak detection.
Namely, while the leak sensor cannot detect a leaked liquid unless it reaches substantially the center part thereof, i.e., a location under the light-transmitting part, if the casing is installed as it is in the subject of leak detection, then the light-transmitting part may attach to the region subjected to leak detection without substantially forming a space therebetween, thus making it hard for the leaked liquid to reach the location under the light-transmitting part.
Also, even when the leak sensor is held so as to form a space from the subjected of leak detection, if the manner of holding is unfavorable, e.g., in a state where the leak sensor is installed such that the light-transmitting part is inclined with respect to the subject of leak detection or the leak sensor is shaky, then the leaked liquid cannot be detected stably.
On the other hand, if the leak sensor is held with a large contact area or too much force so as not to become shaky, the leaked liquid cannot be detected accurately, because of the fact that the light-transmitting part is hidden, distortions and the like occur, and so forth.